This invention relates to aerosol containers and more particularly to a novel aerosol container structure having a novel hermetic sealing arrangement between a valve mounting cup and aerosol container body, and a novel method of providing a hermetic seal between a valve mounting cup and a container body.
Aerosol containers are normally filled with a predetermined amount of propellant that will ensure the dispensing of substantially all the contents in the container in a reasonably constant and acceptable fashion. During normal usage of the aerosol container there is a gradual but expected loss of pressure due, for example, to depletion of propellant. However such propellant depletion generally does not prevent a substantially complete evacuation of the propellable contents because the amount of propellant initially provided in the aerosol container compensates for pressure losses that occur during normal usage of an aerosol container.
Abnormal depletion of propellant due to uncontrolled leakage from the aerosol container may prevent complete dispensing of the container contents, or may cause the dispensed product to have unacceptable characteristics.
Although aerosol containers are routinely tested for leakage after being filled, some containers often leak at a slow enough rate to avoid detection. A common leakage site is through a seam or joint of the dispenser that is designed to provide a hermetic seal. Such leakage, after a predetermined amount of time, can result in a substantial amount of propellant loss that prevents complete emptying of the container contents, or causes the dispensed product to have irregular or unacceptable characteristics.
A major source of potential leakage of propellant in an aerosol dispenser is at the seam or joint where the mounting cup of the aerosol valve is joined to the container by a process known as crimping.
Occasionally an aerosol container that does not leak immediately after assembly will develop a substantial leakage condition within three to six months after packaging, and is often referred to as a latent leakage condition. Latent leakage is generally due to relaxation of the metal at a seam or joint of the container.
As noted at page 395 of The Aerosol Handbook, 2nd Edition, by Montfort A. Johnsen, published by Wayne Dorland Company of Mendham, N.J. "crimping" is normally thought of as the outward spreading of collet segments or jaws to make a hermetic seal whereas "clinching" is a term reserved for the inward compression of valve ferrules by means of collet tines to make an airtight seal. The terms "crimping" and "clinching" as used herein will be in accordance with the foregoing definitions.
Thus, in the formation of a crimp between the valve mounting cup and the container, the valve mounting cup is positioned within the container opening. A generally annular crimping tool having a plurality of crimping fingers in circumferential side-by-side arrangement is radially expanded against the mounting cup. A cup gasket, provided between the mounting cup and the container, helps assure the integrity of the crimped seal.
Examples of a crimped joint between a valve mounting cup and a container are shown in U.S. Pat. Nos. 3,757,717; 4,000,708 and 4,423,823. It will be noted that the opening of the container body which receives the mounting cup usually has a rolled over peripheral curl or bead. It has been found that the surface of this bead often includes radial striations if the container is formed of aluminum.
Referring again to The Aerosol Handbook, page 282, aluminum cans present special problems due to a wide variation in bead contours and the radial drawing grooves (striations, eyelashes) which often cause leakage to occur. The leakage problem intensifies as container diameters increase.
The eyelashes or striations along the bead surface function as leakage channels that can cause a slow but progressive loss of pressure from an aerosol container. Since such leakage is often undetectable during leakage testing, it is troublesome to manufacturers and consumers because spontaneous loss of propellant leads to consumer dissatisfaction with the product.
A further problem which can cause leakage in aluminum aerosol containers is that cans made by different manufacturers often have significant variations in the dimensions and geometry of the container opening. Such variations in the dimensions of the container opening and the geometry of the bead or curl often make it necessary for packagers to modify crimping dimensions according to the supplier of the aerosol container.
The springiness or elasticity of an aluminum valve mounting cup can also adversely affect the integrity of a crimped seal, especially after the hold-down force is removed, following a crimping operation. Gradual slippage may then occur between the surfaces of the valve mounting cup and the gasket to expose a leakage path.
When noncondensable gas propellants are used, such as nitrogen, the problem of propellant leakage through a joint or seal may be expected to occur with increasing frequency and severity, due to their small molecular size and much higher pressures. Leakage is especially critical with such propellants because as little as 0.1 gram of gas can generate the required 110 psig of pressure in these aerosols.
U.S. Pat. No. 3,757,717 attempts to improve the seal at the crimp by providing a crimping collet with inner and outer crimping segments. However the presence of striations in the bead of the container body can still provide a flow path for propellant leakage.
Thus far there are no known arrangements or methods for substantially improving the integrity of the seal provided by a crimped joint between the valve mounting cup and the container.
It is thus desirable to provide an improved joint and method for sealing a valve mounting cup to a container, where such a seal is substantially leak tight, despite the presence of striations in the bead of the container, slight deviations in the dimensional or geometrical characteristics of the container bead and springiness of the bead and the valve mounting cup material.